There are a few option there. If you have forked the xunit reporter you may want to add the dependency to package.json as a git url. This will allow Jenkins to pull down the modified version of the dependency when it pulls down all the other dependencies.
If you just want to include the modified files in the node_modules folder, you can manually add them which will override the .gitignore. Something like the following should work.
git add ./node_modules/customized_package/* -f
The -f will force files to be added, even if they would have otherwise been ignored.